| From: | Doug McNaught <doug(at)wireboard(dot)com> |
|---|---|
| To: | mlw <markw(at)mohawksoft(dot)com> |
| Cc: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>, Andrew Sullivan <andrew(at)libertyrms(dot)info>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Subject: | Re: Index Scans become Seq Scans after VACUUM ANALYSE |
| Date: | 2002-04-17 21:55:14 |
| Message-ID: | m3ofging71.fsf@varsoon.wireboard.com |
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| Thread: | |
| Lists: | pgsql-hackers |
mlw <markw(at)mohawksoft(dot)com> writes:
> It took a bike ride to think about this one. The supposed advantage of a
> sequential read over an random read, in an active multitasking system, is a
> myth.
Disagree.
> Execute a number of queries at the same time, the expected benefit of a
> sequential scan goes out the window. The OS will be fetching blocks, more or
> less, at random.
If readahead is active (and it should be for sequential reads) there
is still a pretty good chance that the next few disk blocks will be in
cache next time you get scheduled.
If your disk is thrashing that badly, you need more RAM and/or more
spindles; using an index will just put even more load on the i/o
system.
-Doug
--
Doug McNaught Wireboard Industries http://www.wireboard.com/
Custom software development, systems and network consulting.
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